Method of crucible-free zone melting a crystalline rod with laterally displaced rod holders

ABSTRACT

METHOD OF CRUCIBLE-FREE ZONE MELTING A CRYSTALLINE ROD WHICH COMPRISES ROTATING A PAIR OF SPACED AND SUBSTANTIALLY VERTICALLY ALIGNED END HOLDERS SUPPORTING A CRYSTALLINE ROD THEREBETWEEN, HEATING THE ROD WITH AN ANNULAR HEATING DEVICE SURROUNDING THE ROD TO A TEMPERATURE AT WHICH A MOLTEN ZONE IS FORMED IN THE ROD DIVIDING THE ROD INTO A SUPPLY ROD PORITION BEING SUPPLIED TO THE MELT IN THE MOLTEN ZONE AND A ROD PORTION RESOLIDIFYING FROM THE MELT, RELATIVELY DISPLACING THE END HOLDERS AND THE HEATING DEVICE IN THE DIRECTION OF THE ROD AXIS AT A GIVEN RELATIVE SPEED SO THAT THE RESOLIDIFYING ROD PORTION HAS A THICKNESS OF PREDETERMINED NOMINAL VALUE, AND LATERALLY DISPLACING THE END HOLDER FOR THE SUPPLY ROD PORTION OUT OF VERTICAL ALIGNMENT WITH THE OTHER END HOLDER.

INE

W. KELLER July 20, 1971 E ZONE MELTING A caYs'r'ALL ROD WITH LATERALLY DISPLACED ROD HOLDERS METHOD OF CRUCIBLE--FRE Filed Sept. 27, 1967 United States Patent Office 3,594,132 Patented July 20, 1971 3,594,132 METHOD OF CRUCIBLE- FREE ZONE MELTING A CRYSTALLINE ROD WITH LATERALLY DIS- PLACED ROD HOLDERS Wolfgang Keller, Pretzfeld, Germany, assignors to Siemens Aktiengesellschaft, Berlin, Germany Filed Sept. 27, 1967, Ser. No. 670,893 Claims priority, application Germany, Sept. 28, 1966, S 106,159 The portion of the term of the patent subsequent to Dec. 3, 1985, has been disclaimed Int. Cl. B01j 17/10 US. Cl. 23-301 Claims ABSTRACT OF THE DISCLOSURE Method of crucible-free zone melting a crystalline rod which comprises rotating a pair of spaced and substantially vertically aligned end holders supporting a crystalline rod therebetween, heating the rod with an annular heating device surrounding the rod to a temperature at which a molten zone is formed in the rod dividing the rod into a. supply rod portion being supplied to the melt in the molten zone and a rod portion resolidifying from the melt, relatively displacing the end holders and the heating device in the direction of the rod axis at a given relative speed so that the resolidifying rod portion has a thickness of predetermined nominal value, and laterally displacing the end holder for the supply rod portion out of vertical alignment with the other end holder.

My invention relates to method for crucible-free zone melting a crystalline rod, especially a semiconductor rod.

Method for crucible-free zone melting a crystalline rod, especially a semiconductor rod, is known from my patent application Ser. No. 428,933, filed Jan. 29, 1965, now abandoned wherein at least one of the holders is rotatable about the substantially perpendicular axis thereof and the holders are movable relative to one another and to an annular heating device surrounding the rod in the direction of the rod axis at a relative speed causing the thickness of the rod portion, resolidifying from the melting zone formed by the heating device in the rod, to be increased beyond the inner diameter of the annular heating device, the rotary holder of the resolidifying rod portion being displaced laterally relative to the heating device. Such a method permits semiconductor rods, preferably silicon rods, to be produced with even larger cross sec-, tions than by the heretofore known concentric or coaxial zone melting methods i.e. wherein supply rod and resolidifying rod remain coaxial throughout the entire process. The rod portion resolidifying from the melt can be located selectively beneath or above the heating device.

It has already been suggested that this process be modified so as to make it advantageously suitable for producing crystalline rods having a thickness equal to the thickness of the supplied rod portion or less than that of the supplied rod portion, though especially not greater than the inner diameter of the heating device. For this purpose, the relative "speeds of the rod holders and the heating device in the axial direction are suitably coordinated. The method according to the aforementioned earlier proposal is distinguished by the fact that the melting zone is intermixed Well and, accordingly, a comparably uniform specific resistance extends over the entire cross section of the rod.

It has now been found that the aforementioned advantage of a good intermixing of the melting zone and therewith a uniform resistance distribution over the cross section of the rod can be achieved in a completely surprising manner and to a great degree by method deviating from the aforedescribed methods.

It is accordingly an object of my invention to provide method for crucible-free zone melting a crystalline rod, especially a semiconductor rod, differing considerably from the aforementioned previously known methods.

It is a further object of my invention to provide such a process wherein a relatively cooler melting current with increased dopant concentration, which supposedly causes the greatly non-uniform specific resistance distribution over the rod cross section produced with the concentric or coaxial zone melting process, and also with the aforementioned eccentric zone melting operation, although to a very greatly reduced degree indeed, can no longer take place. It is also an object of my invention to so well intermix the melting zone as to obtain a very uniform temperature distribution within the melting zone and thereby effect crystallization of the resolidifying rod portion with negligible thermal stresses and great reduction in the frequency of crystal disturbances or dislocations.

With the foregoing and other objects, in view, I provide, in accordance with my invention, method for crucible-free zone melting a crystalline rod, particularly a semiconductor rod, which comprises rotating a pair of spaced and substantially vertically aligned end holders supporting a crystalline rod therebetween, heating the rod with an annular heating device surrounding the rod to a temperature at which a molten zone is formed in the rod dividing the rod into a supply rod portion being supplied to the melt in the molten zone and a rod portion resolidifying from the melt, relatively displacing the end holders and the heating device in the direction of the rod axis at a given relative speed so that the resolidifying rod portion has a thickness of predetermined nominal value, and lat erally displacing the end holder for the supply rod portion out of vertical alignment with the other end holder.

Thus, in accordance with the invention and in contrast to the known methods and to the earlier suggestions, the resolidifying rod portion is maintained concentric or coaxial to the heating device and is not displaceable laterally. By eccentrically displacing the supply rod, the melting boundary surface or liquid-solid interface, which possesses the shape of a substantially pointed cone and is formed by non-uniform heating and heat removal in the supply rod, is very greatly flattened.

In accordance with further features of the invention, the rod portion supplied to the molten zone is relatively displaced laterally to the heating device such a distance that the center axis thereof is passed at least substantially up to the lateral limit or the periphery of the resolidifying rod portion. By the fact that the center axis of the supply rod is substantially aligned or extends laterally with the peripheral surface of the resolidifying rod portion, it is thereby assured that also for non-uniform melting of the supply rod under certain conditions, the comparatively cool melting current with increased dopant concentration formed thereby cannot impinge on the resolidifying rod portion in a direct path. On the contrary, the cool melting current is greatly mixed with the uniformly heated portion of the melting zone due to the particularly strong mixing action at the edge of the melting zone, before the impingement thereof on the recrystallizing boundary surface or the interface between the melt and the resolidifying rod portion.

Other features which are considered as characteristic for the invention are set forth in the appended claims.

Although the invention is illustrated and described herein as method of crucible-free zone melting a crystalline rod, especially a semiconductor rod, it is nevertheless not intended to be limited to the details shown, since various modifications may be made therein Without departing from the spirit of the invention and within the scope and range of equivalents of the claims.

The method of the invention, however, together with additional objects and advantages thereof, is best understood from the following description when read in connection with the accompanying drawing, in which FIGS. 1 to 3 are longitudinal diagrammatic views of a crystalline rod and heating device during three different phases of the method of my invention.

Referring now to the drawing, and first particularly to FIG. 1, thereof, there is shown a lower end of a rodshaped semiconductor member 1, preferably a silicon rod, at which a conical taper 4 is located, to which a thin, rodshaped seed crystal 2 is being fused. The seed crystal 2 is a monocrystal and serves as a monocrystal grower or breeder. The rod-shaped semiconductor member 1 can consist of polycrystalline material and advantageously has a diameter which is greater than the inner diameter of the heating device 3. An induction heating coil energized with high-frequency current is preferably employed as the heating device 3. The conical taper 4 of the semiconductor rod 1 can be produced for example by mechanical processing such as by grinding or sand blasting. It can, however, also be produced by chemical means, for example by etching or melting thereof. The end or point of the conical taper 4 is heated to melting temperature and the seed crystal 2 is then immersed in the melt 5 forming at the tip or point of the taper 4 and is fused thereto. The holder 7 supporting the rod 1 at the upper end thereof and the holder 8 supporting the seed crystal 2 at the lower end thereof, both as shown in FIG. 1, are rotated advantageously in the same rotary direction as indicated by the curved arrows in FIG. 1.

In FIG. 2 the phase of the method of my invention is represented wherein the rod portion 6 resolidifying from the melting zone 5 has achieved its nominal diameter. The nominal diameter of the resolidifying rod portion 6 can be selectively controlled within wide limits by the coordination of the velocities at which the rod portions 1 and 6 are fed in the axial direction thereof. The seed crystal 2 and therewith also the rod portion 6 resolidifying from the melt 5 are disposed concentric or coaxial to the heating device 3. As soon as the resolidifying rod portion 6 attains its nominal diameter, which is advantageously smaller than the inner diameter of the heating device 3, the rod portion 1 supplied to the melt is laterally displaced relative to the heating device.

In FIG. 3 there is shown the final phase of the lateral displacement of the supply rod 1. In the position illustrated in FIG. 3, the center axis M of the rod portion 1 supplied to the melting zone 5 is displaced laterally such a distance that it is aligned with or laterally extends with substantially the lateral limit or periphery of the resolidifying rod portion 6'. As can be seen in FIG. 3, the melted boundary surface or interface between the melt 5 and the solid supply rod 1 shown by the dotted line in FIG. 3, has a relatively flat appearance. It has somewhat the profile of a flat plate. This is attributed to the fact that, due to the illustrated eccentric position of the supply rod 1, the heating device 3 exerts very intense heating action on the central interior region of the supply rod, in contrast to the known concentric or coaxial zone melting process, wherein the heating action in the region of the center axis of the supply tube 1 is comparatively small. The uniform intermixing and/ or heating of the melt can be promoted further by periodically reciprocating the holder of the rod portion 1 supplied to the melting zone 5 in the eccentric position about its middle axis M or to subject it to eccentric motion about an axis M. The amplitude of the linear and of the eccentric motion is therewith advantageously established at about a half radius of the resolidifying rod portion. The heating coil 3 advantageously has such a large number of windings as will cause the peripheral surface of the rod portion 1 being supplied to the melting zone 5 and the outer diameter of the heating device 3 in the eccentric position of the rod 1 to become substantially aligned with one another. The melting zone 5 thereby becomes particularly stable.

The aforedescribed method can be modified so that the concentric or coaxial resolidifying rod portion 6 is located above the heating coil 3 and the eccentric rod portion 1 supplied to the melting zone is located below the heating coil 3. The method of my invention is not limited to the production of monocrystalline rods having a diameter smaller than or the same as the inner diameter of the heating device. 7

I claim: a

1. Method of crucible-free zone melting a crystalline rod which comprises rotating a pair of spaced and substantially vertically aligned end holders supporting a crystalline rod therebetween, heating the rod with an annular heating device surrounding the rod to a temperature at which a molten zone is formed in the rod dividing the rod into a supply rod portion being supplied to the melt in the molten zone and a rod portion resolidifying from the melt, relatively displacing the end holders and the heating device in the direction of the rod axis at a given relative speed so that the resolidifying rod portion has a thickness of predetermined nominal value, and laterally displacing the end holder for the supply rod portion out of vertical alignment with the other end holder.

2. Method according to claim 1 wherein the end holder for the supply rod portion is laterally displaced relative to the heating device to a location at which the center axis of the end holder for the supply rod portion is at least substantially aligned with the lateral limit of the resolidifying rod portion.

3. Method according to claim 1 wherein the supply rod portion has a thickness that is at least equal to the inner diameter of the annular heating device and the displacement speeds of the end holder for the supply rod portion and of the end holder for the resolidifying rod portion in the axial directions thereof are of such values that the thickness of the resolidifying rod portion is at most equal to the inner diameter of the annular heating device.

4. Method according to claim 1 which further comprises periodically subjecting to an eccentric movement about the center thereof, the holder of the supply rod portion which has been laterally displaced out of vertical alignment with the holder of the resolidifying rod portion.

5. Method according to claim 1 which further comprises periodically subjecting to an eccentric movement about the center axis thereof, the holder of the supply rod portion which has been laterally displaced out of vertical alignment with the holder of the resolidifying rod portion.

References Cited UNITED STATES PATENTS 3,228,753 1/1966 Larsen 23-273 3,296,036 1/1967 Keller 23-301X 3,414,388 12/1968 Keller 23--301 FOREIGN PATENTS 882,570 11/1961 Great Britain 23-301 NORMAN YUDKOFF, Primary Examiner R. T. FOSTER, Assistant Examiner 11.5. C1. X.R. 23-273 

